The present invention relates to a power control method used in a code-division multiple access (CDMA) format communication system, which is particularly suitable for application to a communication system having a multiple access interference (MAI) canceller capability.
Generally, in CDMA radio communication formats such as wideband CDMA (W-CDMA), each uplink from a mobile station (MS) to a base station (BS) has two power control systems known as the outer loop and the inner loop. The outer loop is a higher layer control for independently adjusting the target for uplink power control in each cell in an active group. Since the outer loop is based on measurement results of a block error rate (BLER) of decoded data, the control response is delayed by the transmission time interval (TTI) required for calculating the block error rate.
The inner loop, to which the present invention is primarily directed, makes minor adjustments of the mobile station transmission power to hold the signal-to-interference ratio (hereinafter referred to as SIR) of the uplink at a specific target. The inner loop power control of the uplink is performed by the following method (see 3rd Generation Partnership Project (3GPP) TS 25.214, v1.1.0, UTRA FDD Physical Layer Procedures):    (1) The base station receives a dedicated physical control channel (DPCCH) of an uplink, and measures the SIR value of the received signal.    (2) Next, the base station compares the measured SIR value to a target value for the signal-to-interference power ratio (hereinafter referred to as SIR target value), and determines a transmission power control (TPC) command for the uplink in order to control the transmitting power of the mobile station.    (3) The transmission power control command which has been determined in this way is next inserted into a predetermined position in the slot of the downlink (the communication in the direction from the base station to the mobile station) transmitted immediately after the slot which is currently being transmitted, and transmitted to the mobile station.    (4) The mobile station adjusts the transmission power of the uplink in accordance with the received transmission power control command.
Since response delays of the power control degrade the system performance, the inner loop of the power control format was performed for each slot period and SIR measurement was performed in real-time in conventional methods.
The interference canceller (IC) has been proposed as art for increasing system capacity by eliminating multiple access interference which occurs in CDMA communications. In base stations having such interference canceling capabilities, the multiple access interference components are subtracted from the received CDMA signals by means of an interference canceling unit (ICU) of each uplink channel. When a desired dedicated physical data channel (DPDCH) is demodulated at the base station, the multiple access interference components can be substantially removed from the received signal by means of an iterative interference subtractive operation of a multistage interference canceling unit. As a result, it is possible to improve the SIR vale of a desired dedicated physical data channel, thus further increasing the system capacity. This is explained in many technical papers and articles such as A. Duel-Hallen et al., “Multiuser Detection for CDMA Systems,” IEEE Personal Communications, pp. 46–58, April 1995 and S. Moshavi, “Multi-user Detection for DS-CDMA Communications,” IEEE Communications Magazine, pp. 124–136, October 1996.
In normal base stations having an interference canceling capability, SIR measurements of the dedicated physical control channel are made prior to interference cancellation. As a result, the measured SIR value is smaller than the SIR value of the dedicated physical data channel after interference cancellation, which is demodulated and decoded. For this reason, if transmission power control of the uplink is performed based on the SIR value prior to interference cancellation in accordance with the transmission power control method described above, there is a risk of the transmission power of the uplink being unnecessarily raised. In order to avoid this, one might conceive of performing transmission power control based on the SIR value after interference cancellation, but in this case, the power control is delayed because of the time required for the interference cancellation operation.
Additionally, the SIR target value used to determined the transmission power control command is determined by the outer loop based on the block error rate as described above. Since the block error rate is calculated from the results of a cyclic redundancy check (CRC), it cannot be calculated until the decoding operation (rate de-matching de-interleaving, channel decoding and CRC judgment) is completed with respect to the entire frame. Furthermore, in order to measure the block error rate (e.g. 20 ms−2 s), it is necessary to perform cyclic redundancy checks of a plurality of frames. Therefore, the SIR target values from the outer loop are delayed by the block error rate measurement.
This response delay in the power control is one factor in the degradation of the system capacity. This effect is particularly apparent when the state of the communication path suddenly changes, such as when connecting or terminating new channels (calls) or due to fast fading effects.